1. Field of the Invention
The invention generally pertains to apparatus for supplying electrical energy to synchronous stepper motors. More specifically, the invention relates to a chopper controlled drive circuit for a permanent magnet step motor.
2. Description of the Prior Art
The step motor is a synchronous machine designed to rotate its rotor a predetermined amount in response to each electrical pulse, or step command, received by its drive circuit. The rotation is effected by delivering appropriate drive currents in response to received step commands to sequentially energize selected stator windings of the motor to force the axis of the air gap between rotor and stator poles into alignment. The portion of the motor drive furnishing the proper drive current switching sequence among the stator windings is commonly referred to as the translator section.
A known prior art approach utilizes two windings on each stator section of equal turns but of opposite winding sense (e.g., so-called "bifilar" windings) to effect reversal of magnetic flux in stator pole pieces without the need for two separate voltage sources of opposite polarity.
Further details of step motor types and typical prior art approaches to providing current drives thereto are found in a series of technical papers available from the Superior Electric Company, Bristol, Conn. 06010.
In a typical bifilar wound, permanent magnet rotor step motor, two bifilar wound coils (four coils total) are alternatively switched to provide stepping action. It is a further known technique to attempt to optimize step motor performance by furnishing a substantially constant current to stator windings in the course of energizing the motor. One such known approach is the so-called chopper driver. The typical chopper driver for a step motor utilizes a current sensor at the stator windings whose output is compared to a reference level. When stator current exceeds the reference level the chopper operates to interrupt the stator current supply until coil current decays to a level just below the reference, at which time the stator winding current source is reactivated.
Prior chopper controlled drive arrangements have raised problems of motor heating and heating of drive components due to relatively high chop rates, slow commutating action due to inductive loading during stator winding phase switching, and transient current spikes causing excessive power dissipation when switching between phases.